The recent isolation of a new serotype of Bacillus thuringiensis provides the first biological control agent suitable for use against black fly larvae. Like the strains of B. thuringiensis used against agricultural and forest pests, B. thuringiensis var. israelensis (serotype 14) derives its insecticidal activity largely from the production of proteinaceous crystals. Morphologically and biochemically, the new serotype resembles other B. thuringiensis strains, although the toxic crystals are pleomorphic in shape and size. More significantly, variety israelensis is unusual in being relatively ineffective against lepidopterous insects, while being highly lethal to some dipterans, including black flies. Its high efficacy and rapid action against a wide spectrum of black flies have been repeatedly demonstrated in laboratory and field tests. Greatest efficacy has been noted in tests at warm water temperatures and against early-instar larvae. Poor carry in low discharge streams has limited treatment effectiveness, but excellent results have been achieved in larger streams. Field trials have indicated that the bacterium is innocuous to nontarget stream organisms, but further evaluation of the direct and indirect effects of treatment on the stream environment is suggested. Although additional research is also needed concerning such parameters as formulation, carry and the relationship between concentration and exposure time, the available evidence strongly supports the use of B. thuringiensis var. israelensis as an alternate to the chemical larvicides currently in use for black fly control.